The invention relates to a picture signal processing circuit for processing a picture signal having picture information components at different signal levels. The majority of the signal level differences are greater than the picture information differences at the signal levels. The circuit is formed with an amplifier circuit having two parallel, first and second signal channels between an input and an output and a change-over circuit via which each time one of the two parallel signal channels of the amplifier circuit is operative for signal processing. The signal channels of the amplifier circuit are formed with amplifiers each having a different transmission characteristic.
A picture signal processing circuit of this type is known from British Pat. No. 1,181,758, more specifically for use in infrared picture signal processing. In thermal picture recording, scene components having relatively large temperature differences relative to the background may occur against a background of relatively small temperature differences. Consequently, the picture signal is obtained with the picture information components at different signal levels, the signal level differences being greater than the picture information differences at the signal levels. The generated picture signal has such a large signal range that the amplifier circuit and other circuits are overdriven without signal-limiting measures. To avoid overdrive the use is possible of the amplifier circuit with the two parallel signal channels and the amplifiers having the different transmission characteristics and the change-over circuit.
It has been described that it is possible to use a normal picture signal amplifier in combination with an amplifier which is only operative for a picture signal giving information around a given signal level which is located between two pre-determined values which correspond to the limit values of a given temperature range, that is to say, located in a given thermal band. The thermal band may be adjusted, as desired, within the total temperature range. The problem has been described that without further measures no distinction can be made between information from within and outside the thermal band when the obtained picture signal combination is displayed. To solve this problem, it is proposed to periodically switch the change-over circuit in such a way that the thermal band information is displayed with a periodical duration which is different from the information occurring outside the thermal band. The distinction in information thereby becomes manifest in a periodically longer or shorter display time. It has been described that such a display is advantageous in the case of a photographic recording of the displayed picture due to the longer or shorter exposure time.
In the case of display of a picture which can be observed directly, the solution described leads to an unacceptable picture in which a flickering phenomenon is involved in the distinction of information.